Verify and decode a signed snapshot blob

Accepts a base64-encoded snapshot blob previously produced by POST /snapshot/save, verifies its HMAC-SHA256 signature against TENSOR_WASM_API_SNAPSHOT_HMAC_KEY, and returns the HMAC-authenticated provenance (captured tenant, instance id, uncompressed size, wire version).

Hardening: the reader always requires a signature (require_signature) so a stripped-trailer downgrade to an unsigned v2 blob is refused; TENSOR_WASM_API_SNAPSHOT_REQUIRE_SIGNATURE is the operator surface for this posture. A 256 MiB decompression cap bounds restore-time memory pressure.

Authorization: protected stack (bearer auth + tenant scope). After HMAC verification recovers the snapshot's captured tenant, the handler enforces that it equals the caller's resolved tenant — a cross-tenant restore is 403 tenant_scope_denied. A wrong / missing key or a tampered blob is 403 snapshot_signature_invalid (a single opaque rejection, not a decode oracle).

NOTE: this verifies and decodes the snapshot envelope. Reconstituting a live running instance from the captured memory needs an executor restore hook that does not exist yet; requesting that capability surfaces 501 not_implemented.